Fuel cell system and method of controlling the same

ABSTRACT

A controller of a fuel cell system performs cathode gas supply control to raise an average cell voltage of a fuel cell stack by increasing supply of cathode gas to the fuel cell stack, when electric power required to be generated by the fuel cell stack is equal to zero, and the average cell voltage is lower than a predetermined target voltage. Under the cathode gas supply control, the controller sets the target voltage when a predetermined condition indicating that crossleak is likely to occur is satisfied, to a value higher than a reference target voltage as the target voltage in the case where the condition is not satisfied.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2017-216004 filed onNov. 9, 2017 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The disclosure relates to a fuel cell system and a method of controllingthe fuel cell system.

2. Description of Related Art

In a fuel cell system, when a gas passage is blocked with water produced(namely, when flooding occurs), a voltage value of each unit cell (whichwill be called “cell voltage”) is reduced, as known in the art. In afuel cell system described in Japanese Unexamined Patent ApplicationPublication No. 2012-227008 (JP 2012-227008 A), when the lowest cellvoltage becomes lower than a predetermined value, the amount of cathodegas supplied to the cells is increased, so as to eliminate the flooding,and thus recover the cell voltage.

SUMMARY

In the meantime, when a fuel cell stack is not required to generateelectric power, it is common to reduce the flow rate of cathode gassupplied to the fuel cell stack, or stop the supply. If permeation ofanode gas to the cathode side (so-called “crossleak”) occurs, in acondition where the flow rate of cathode gas supplied is reduced, thecell voltage drops. The inventors of this application found that, ifcrossleak occurs during a period immediately after the fuel cell stackceases to be required to generate electric power, and the flow rate ofcathode gas supplied is reduced, the gas pressure on the cathode side isreduced, and the cell voltage is rapidly reduced accordingly. In thiscase, even if the flow rate of cathode gas supplied is increased afterdetection of the crossleak, the cell voltage cannot be recovered intime, and may be reduced sharply to a voltage at which the cellsdeteriorate. Thus, a technology for preventing the cell voltage frombeing excessively reduced is desired.

A first aspect of the disclosure is concerned with a fuel cell systemincluding a fuel cell stack having a plurality of unit cells, an anodegas supply unit that supplies anode gas to the fuel cell stack, acathode gas supply unit that supplies cathode gas to the fuel cellstack, a voltage detector that detects a voltage of the fuel cell stack,and a controller that controls the anode gas supply unit and the cathodegas supply unit. The controller performs cathode gas supply control toraise an average cell voltage of the fuel cell stack by increasingsupply of the cathode gas to the fuel cell stack by the cathode gassupply unit, when electric power required to be generated by the fuelcell stack is equal to zero, and the average cell voltage is lower thana predetermined target voltage. Under the cathode gas supply control,the controller determines whether a predetermined condition indicatingthat crossleak is likely to occur is satisfied, and sets the targetvoltage when the predetermined condition is satisfied, to a value thatis higher than a reference target voltage as the target voltage in acase where the predetermined condition is not satisfied. The crossleakrepresents permeation of the anode gas from an anode electrode to acathode electrode in each of the unit cells. With the fuel cell systemthus configured, the cathode gas supply control is performed by settingthe target voltage to a high value when the crossleak occurrencecondition is satisfied. Thus, the flow rate of cathode gas supplied isincreased at an earlier point in time, and the cell voltage can be madeless likely or unlikely to be excessively reduced.

The fuel cell system may further include a pressure measuring unit thatmeasures an anode gas pressure of the fuel cell stack. The predeterminedcondition may include a condition that the anode gas pressure is higherthan a predetermined threshold pressure. With the fuel cell system thusconfigured, it is possible to easily detect a condition where crossleakis likely to occur.

Under the cathode gas supply control, the controller may set the targetvoltage to a higher value when the anode gas pressure is higher than thethreshold pressure, than the target voltage in a case where the anodegas pressure is lower than the threshold pressure. With the fuel cellsystem thus configured, the cell voltage can be effectively made lesslikely or unlikely to be excessively reduced.

The predetermined condition may include a condition that the electricpower required to be generated by the fuel cell stack immediately beforethe required electric power is reduced to zero is equal to or large thana predetermined threshold power. With the fuel cell system thusconfigured, it is possible to easily detect a condition where crossleakis likely to occur.

A second aspect of the present disclosure relate to a method ofcontrolling a fuel cell system having a fuel cell stack having aplurality of unit cells. The method includes performing cathode gassupply control to raise an average cell voltage of the fuel cell stackby increasing supply of cathode gas to the fuel cell stack, whenelectric power required to be generated by the fuel cell stack is equalto zero, and the average cell voltage is lower than a predeterminedtarget voltage. Under the cathode gas supply control, it is determinedwhether a predetermined condition indicating that crossleak is likely tooccur is satisfied, and the target voltage is set when the predeterminedcondition is satisfied, to a value that is higher than a referencetarget voltage as the target voltage in a case where the predeterminedcondition is not satisfied. The crossleak representing permeation ofanode gas from an anode electrode to a cathode electrode in each of theunit cells of the fuel cell stack.

The disclosure can be implemented in various forms. For example, thedisclosure may be implemented in the form of a power generationapparatus including the fuel cell system, a vehicle including the fuelcell system, and so forth.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the disclosure will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a schematic view schematically showing the configuration of afuel cell system;

FIG. 2 is a flowchart illustrating one example of the procedure ofcathode gas supply control;

FIG. 3 is a flowchart illustrating one example of the procedure of atarget voltage determining process;

FIG. 4 is a view showing the relationship among the anode gas pressure,average cell voltage, and indicated flow rate of cathode gas;

FIG. 5 is a flowchart illustrating one example of the procedure of atarget voltage determining process according to a second embodiment;

FIG. 6 is a graph showing the relationship between the anode gaspressure and the target voltage according to a third embodiment;

FIG. 7 is a graph showing the relationship between the anode gaspressure and the target voltage according to a fourth embodiment;

FIG. 8 is a graph showing the relationship between the anode gaspressure and the target voltage according to a fifth embodiment; and

FIG. 9 is a graph showing the relationship between the anode gaspressure and the target voltage according to a sixth embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS A. First Embodiment

FIG. 1 schematically shows the configuration of a fuel cell system 100according to one embodiment of the disclosure. The fuel cell system 100includes a fuel cell stack 10, controller 20, cathode gas supply unit30, anode gas supply unit 50, and cooling medium circulating unit 70.The fuel cell system 100 also includes a DC/DC converter 80, powercontrol unit (which will be referred to as “PCU”) 81, load 82, impedancemeasuring unit 83, and voltage detector 84. The fuel cell system 100 ofthis embodiment is installed on a fuel cell vehicle, for example.

The fuel cell stack 10 is a polymer electrolyte fuel cell that issupplied with anode gas (e.g., hydrogen gas) and cathode gas (e.g., air)as reaction gases, to generate electric power. The fuel cell stack 10 iscomposed of a plurality of unit cells 11 stacked together. Each unitcell 11 has a membrane electrode assembly (not shown) in which an anodeelectrode (not shown) and a cathode electrode (not shown) are disposedon opposite surfaces of an electrolyte membrane (not shown), and a pairof separators (not shown) between which the membrane electrode assemblyis sandwiched.

The controller 20 is configured as a computer including a centralprocessing unit (CPU), a memory, and an interface circuit to whichrespective components that will be described later are connected. Thecontroller 20 outputs a signal for controlling start and stop of eachdevice in the fuel cell system 100, according to a command of anelectronic control unit (ECU) 21. The ECU 21 is a controller thatcontrols the whole apparatus (e.g., vehicle) including the fuel cellsystem 100. In the fuel cell vehicle, for example, the ECU 21 controlsthe vehicle, according to a plurality of input values, such as theamount of depression of an accelerator pedal, the amount of depressionof a brake pedal, and the vehicle speed. The ECU 21 may be included as apart of the functions of the controller 20. The CPU executes controlprograms stored in the memory, so as to control power generation by thefuel cell system 100, and implement cathode gas supply control that willbe described later.

The cathode gas supply unit 30 includes a cathode gas pipe 31, air flowmeter 32, air compressor 33, first shutoff valve 34, pressure gauge 35,shunt valve 36, cathode offgas pipe 41, and first regulator 42. Thecathode gas pipe 31 is connected to the fuel cell stack 10, and suppliesair taken in from the outside, to the fuel cell stack 10.

The air flow meter 32 is provided in the cathode gas pipe 31, andmeasures the flow rate of air taken into the cathode gas pipe 31. Theair compressor 33 compresses air taken in from the outside, according toa control signal from the controller 20, and supplies the compressed airas cathode gas, to the fuel cell stack 10. The first shutoff valve 34 isprovided between the air compressor 33 and the fuel cell stack 10. Thepressure gauge 35 measures the pressure (which will be referred to as“cathode gas pressure”) at a cathode gas inlet of the fuel cell stack10, and sends the result of measurement to the controller 20. The shuntvalve 36 is provided between the air compressor 33 and the cathodeoffgas pipe 41, and adjusts the flow rates of air into the fuel cellstack 10 and the cathode offgas pipe 41.

The cathode offgas pipe 41 discharges cathode offgas discharged from thefuel cell stack 10, to the outside of the fuel cell system 100. Thefirst regulator 42 controls the pressure at a cathode gas outlet of thefuel cell stack 10, according to a control signal from the controller20.

The anode gas supply unit 50 includes an anode gas pipe 51, anode gastank 52, second shutoff valve 53, second regulator 54, injector 55,pressure gauge 56, anode offgas pipe 61, gas-liquid separator 62,air-water discharge valve 63, circulation pipe 64, and anode gas pump65. In the following description, a passage including a portion of theanode gas pipe 51 located downstream of the injector 55, passage ofanode gas in the fuel cell stack 10, anode offgas pipe 61, gas-liquidseparator 62, circulation pipe 64, and anode gas pump 65, will be called“circulation passage 66”. The circulation passage 66 is a passagethrough which anode offgas of the fuel cell stack 10 is circulated backto the fuel cell stack 10.

The anode gas tank 52 is connected to an anode gas inlet of the fuelcell stack 10 via the anode gas pipe 51, and supplies anode gas to thefuel cell stack 10. The second shutoff valve 53, second regulator 54,injector 55, and pressure gauge 56 are provided in the anode gas pipe51, in this order as viewed from the upstream side, namely, from theside closer to the anode gas tank 52.

The second shutoff valve 53 is opened and closed according to a controlsignal from the controller 20. When the fuel cell system 100 is stopped,the second shutoff valve 53 is closed. The second regulator 54 controlsthe pressure of hydrogen at the upstream side of the injector 55,according to a control signal from the controller 20. The injector 55 isan electromagnetically driven shutoff valve having a valve body that iselectromagnetically driven, according to a drive cycle and avalve-opening period set by the controller 20. The controller 20controls the flow rate of anode gas supplied to the fuel cell stack 10,by controlling the drive cycle and valve-opening period of the injector55. The pressure gauge 56 measures the pressure at the anode gas inletof the fuel cell stack 10, and sends the result of measurement to thecontroller 20. The pressure gauge 56 may be provided at an anode gasoutlet side of the fuel cell stack 10. In this case, the above-mentionedpressure gauge 35 is also preferably provided at the cathode gas outletside of the fuel cell stack 10. In either case, the pressure measured bythe pressure gauge 56 may be called “anode gas pressure”. The pressuregauge 56 may also be called “pressure measuring unit”.

The anode offgas pipe 61 connects the anode gas outlet of the fuel cellstack 10, with the gas-liquid separator 62. The anode offgas pipe 61leads anode offgas including hydrogen gas and nitrogen gas that were notused in power generation reaction, to the gas-liquid separator 62.

The gas-liquid separator 62 is connected between the anode offgas pipe61 and circulation pipe 64 of the circulation passage 66. The gas-liquidseparator 62 separates water as an impurity, from the anode offgas inthe circulation passage 66, and stores the water therein.

The air-water discharge valve 63 is provided below the gas-liquidseparator 62. The air-water discharge valve 63 discharges water storedin the gas-liquid separator 62, and discharges unnecessary gas (mainly,nitrogen gas) in the gas-liquid separator 62. During operation of thefuel cell system 100, the air-water discharge valve 63 is normallyclosed, and is opened and closed according to a control signal from thecontroller 20. In this embodiment, the air-water discharge valve 63 isconnected to the cathode offgas pipe 41, and the water and unnecessarygas discharged via the air-water discharge valve 63 are discharged tothe outside, through the cathode offgas pipe 41.

The circulation pipe 64 is connected to a portion of the anode gas pipe51 located downstream of the injector 55. The anode gas pump 65 that isdriven according to a control signal from the controller 20 is providedin the circulation pipe 64. The anode offgas from which water wasseparated by the gas-liquid separator 62 is delivered into the anode gaspipe 51, by means of the anode gas pump 65. In this fuel cell system100, the anode offgas containing hydrogen is circulated, and suppliedagain to the fuel cell stack 10, for improvement of the anode-gas useefficiency.

The cooling medium circulating unit 70 circulates a cooling medium viathe fuel cell stack 10, so as to control the temperature of the fuelcell stack 10. The cooling medium circulating unit 70 includes a coolantsupply pipe 71, coolant discharge pipe 72, radiator 73, coolant pump 74,three-way valve 75, bypass pipe 76, and temperature gauge 77. As acoolant, water, antifreeze liquid, such as ethylene glycol, air, or thelike, may be used.

The coolant supply pipe 71 is connected to a coolant inlet in the fuelcell stack 10, and the coolant discharge pipe 72 is connected to acoolant outlet of the fuel cell stack 10. The radiator 73, which isconnected to the coolant discharge pipe 72 and the coolant supply pipe71, cools the cooling medium flowing in from the coolant discharge pipe72, with air blow of an electric fan, for example, and discharges thecooled medium to the coolant supply pipe 71. The coolant pump 74 isprovided in the coolant supply pipe 71, and feeds the coolant underpressure to the fuel cell stack 10. The three-way valve 75 adjusts theflow rate of the coolant to the radiator 73 and the bypass pipe 76. Thetemperature gauge 77 is connected to the coolant discharge pipe 72, andmeasures the temperature of the coolant discharged from the fuel cellstack 10. The temperature measured by the temperature gauge 77 issubstantially equal to the temperature of the fuel cell stack 10.

The DC/DC converter 80 raises the output voltage of the fuel cell stack10, and supplies it to the PCU 81. The PCU 81 incorporates an inverter,and supplies electric power to the load 82, via the inverter, undercontrol of the controller 20. The PCU 81 also limits electric current ofthe fuel cell stack 10, under control of the controller 20. An ammeter85 that measures the current of the fuel cell stack 10 is providedbetween the fuel cell stack 10 and the DC/DC converter 80.

The voltage detector 84 detects the voltage of the fuel cell stack 10.In this embodiment, the voltage detector 84 calculates the average cellvoltage from the voltage of the fuel cell stack 10. The “average cellvoltage” is a value obtained by dividing a voltage across opposite endsof the fuel cell stack 10 by the number of the unit cells 11.

The ammeter 85 measures the output current value of the fuel cell stack10. The impedance measuring unit 83 measures alternating-currentimpedance of the fuel cell stack 10, using the voltage detector 84 andthe ammeter 85, and sends its measurement value to the controller 20.

Electric power of the fuel cell stack 10 is supplied to the load 82,such as a traction motor (not shown) for driving wheels (not shown), andthe above-mentioned air compressor 33, anode gas pump 65, and variousvalves, via a power supply circuit including the PCU 81.

The flowchart of FIG. 2 illustrates one example of the procedure ofcathode gas supply control according to this embodiment. The routine ofFIG. 2 is started when the fuel cell system 100 shifts from normaloperation to zero required output operation. The “zero required outputoperation” is an operating mode of the fuel cell system 100 establishedwhen electric power which the ECU 21 requires the fuel cell stack 10 togenerate is equal to zero. The “zero required output operation” willalso be called “intermittent operation”. During zero required outputoperation, small current may be generated from the fuel cell stack 10,so as to prevent the voltage of the unit cell 11 from being equal to anopen circuit voltage. In the zero required output operation, electricpower for each device is supplied from another power supply (not shown),such as a secondary battery. The “zero required output operation” alsoincludes the case where electric power generated by the fuel cell stack10 is charged into a secondary battery, or the like, without being usedfor driving the load 82, such as a motor. In the zero required outputoperation, accessories are stopped as far as possible, for improvementof the fuel efficiency. In particular, it is preferable to stop the aircompressor 33 as far as possible during zero required output generation,because a large amount of electric power is consumed by the aircompressor 33.

The controller 20 starts cathode gas supply control shown in FIG. 2 whenzero required output operation starts. The controller 20 finishes thecontrol of FIG. 2, when a command to stop zero required output operationis generated, more specifically, when electric power which the ECU 21requires the fuel cell stack 10 to generate ceases to be zero, namely,when the ECU 21 requires the fuel cell stack 10 to generate electricpower.

In step S100, the controller 20 reduces the flow rate of cathode gassupplied to the fuel cell stack 10. In this embodiment, the aircompressor 33 is stopped, and supply of cathode gas is stopped. Morespecifically, the flow rate of cathode gas supplied to the fuel cellstack 10 by the air compressor 33 is set to zero. It is thus preferableto stop the air compressor 33, in terms of improvement in the fuelefficiency. It is also preferable, in terms of improvement in the fuelefficiency, that the flow rate of anode gas supplied is set to zero.Various methods can be employed, for reducing the flow rate of cathodegas supplied to the fuel cell stack 10, without stopping supply ofcathode gas. For example, the flow rate of cathode gas supplied to thefuel cell stack 10 may be reduced by controlling the opening of theshunt valve 36, while keeping the air compressor 33 in a driven state.

Then, the controller 20 determines a target voltage Vm of the averagecell voltage in step S110. This process will be called “target voltagedetermining process”. The target voltage Vm is preferably set to behigher than 0V, and is more preferably set to a value equal to or higherthan 0.6V and equal to or lower than 0.85V. Details of the targetvoltage determining process will be described later.

Then, in step S120, the controller 20 obtains the average cell voltageVfc, and determines whether the average cell voltage Vfc is lower thanthe target voltage Vm. When the average cell voltage Vfc is lower thanthe target voltage Vm, the controller 20 proceeds to step S130, toincrease the flow rate of cathode gas supplied to the fuel cell stack10. In this embodiment, operation of the air compressor 33 is started,and supply of cathode gas is resumed. The flow rate of cathode gassupplied at this time is empirically determined in advance, and may beset as desired. The flow rate of cathode gas supplied in step S130 mayalso be determined, based on a map or function that defines therelationship between conditions of the fuel cell stack 10 and the flowrate of cathode gas. On the other hand, when the average cell voltageVfc is equal to or higher than the target voltage Vm, the controller 20returns to step S100. Namely, supply of the cathode gas is reduced orkept stopped, until the average cell voltage Vfc becomes lower than thetarget voltage Vm. In this connection, step S110 may be executed onlywhen the routine of FIG. 2 is executed for the first time.

After increasing the flow rate of cathode gas supplied to the fuel cellstack 10 in step S130, the controller 20 obtains the average cellvoltage Vfc again, and determines whether the average cell voltage Vfcis higher than the target voltage Vm. When the average cell voltage Vfcis higher than the target voltage Vm, the controller 20 returns to stepS100, and reduces the flow rate of cathode gas supplied to the fuel cellstack 10. In this embodiment, the controller 20 stops the air compressor33, and stops supply of cathode gas. On the other hand, when the averagecell voltage Vfc is equal to or lower than the target voltage Vm, thecontroller 20 returns to step S140, and continues supply of cathode gas.Thus, in steps S130, S140, the flow rate of cathode gas supplied istemporarily increased until the average cell voltage Vfc exceeds thetarget voltage Vm.

As the target voltage Vm used in step S140, a value higher than thetarget voltage Vm used in step S120 may be used. However, the targetvoltage Vm used in step S140 and the target voltage Vm used in step S120are preferably set to the same value, so that control is moresimplified. Between step S130 and step S140, a process of determining atarget voltage again may be performed.

While the average cell voltage Vfc is used for making determinations insteps S120, S140 in the routine of FIG. 2, the voltage across theopposite ends of the fuel cell stack 10 may be used in place of theaverage cell voltage Vfc, for making determinations in step S120, S140.Since the average cell voltage Vfc is obtained by dividing the voltageacross the opposite ends of the fuel cell stack 10 by the number of theunit cells 11, as described above, the determinations made based on thevoltage across the opposite ends of the fuel cell stack 10 areequivalent to the determinations made based on the average cell voltage.

The flowchart of FIG. 3 illustrates one example of the procedure of thetarget voltage determining process. Initially, the controller 20determines in step S200 whether a condition (which will be referred toas “crossleak occurrence condition”) under which crossleak, i.e.,permeation of anode gas from the anode electrode to the cathodeelectrode in the unit cell 11, is likely to occur is satisfied. As thecrossleak occurrence condition, any of the following conditions may beemployed, for example.

First crossleak occurrence condition: the pressure of anode gas ishigher than a predetermined threshold pressure.

Second crossleak occurrence condition: electric power required to begenerated by the fuel cell system 100 immediately before the zerorequired output operation is started was equal to or larger than apredetermined threshold power.

Third crossleak occurrence condition: the temperature of the fuel cellstack 10 is higher than a predetermined threshold temperature.

Fourth crossleak occurrence condition: the relative humidity of theelectrolyte membrane of the unit cell 11 of the fuel cell stack 10 ishigher than a predetermined threshold humidity.

Fifth crossleak occurrence condition: a difference between the anode gaspressure and the cathode gas pressure is higher than a predeterminedthreshold value.

When the first crossleak occurrence condition is satisfied, the gaspressure on the anode side is high, in each unit cell 11; therefore,crossleak is likely to occur. If the average cell voltage Vfc is reducedin a condition where the anode gas pressure is high, the slope ofreduction is likely to be large. Thus, when the condition where theanode gas pressure is high is used as a crossleak occurrence condition,it can be advantageously and correctly determined whether the cellvoltage is excessively reduced. The threshold pressure can beempirically determined in advance. As the threshold pressure, a value inthe range of 130 kPa to 160 kPa may be used, for example.

When the second crossleak occurrence condition is satisfied, arelatively large amount of anode gas has been supplied to the fuel cellstack 10, so as to generate relatively large electric power.Accordingly, in this case, the gas pressure on the anode electrode sideof each unit cell 11 is high, and crossleak is likely to occur;therefore, the determination in step S200 can be made only based on therequired electric power, without using the anode gas pressure. If theaverage cell voltage Vfc is reduced while the required electric power islarge, the slope of the reduction is likely to be large. Thus, when thecondition where the required electric power is large is used as acrossleak occurrence condition, it can be advantageously and correctlydetermined whether the cell voltage is excessively reduced. Thethreshold power can be empirically determined in advance. As thethreshold power, a value in the range of 10 W to 20 W per unit cell 11may be used, for example.

When the third crossleak occurrence condition is satisfied, thetemperature is high in each unit cell 11; therefore, the velocity ofvibration of molecules increases, and crossleak is likely to occur. Thethreshold temperature can be empirically determined in advance. Forexample, a value in the range of 70° C. to 90° C. may be used as thethreshold temperature.

When the fourth crossleak occurrence condition is satisfied, therelative humidity of the electrolyte membrane is high, in each unit cell11; therefore, the amount of water contained in the electrolyte membraneis large, and anode gas is likely to be dissolved in the water of theelectrolyte membrane. As a result, crossleak is likely to occur. Therelative humidity of the electrolyte membrane can be estimated from thealternating-current (AC) impedance. The AC impedance corresponds to aresistance value of the electrolyte membrane of the unit cell 11, andcorrelates with the water content of the electrolyte membrane. Morespecifically, the impedance value increases as the water content of theelectrolyte membrane is smaller, namely, as the relative humidity of theelectrolyte membrane is lower. On the contrary, the impedance valuedecreases as the water content of the electrolyte membrane is larger,namely, as the relative humidity of the electrolyte membrane is higher.The threshold humidity can be empirically determined in advance. Forexample, a value in the range of 60% to 70% may be used as the thresholdhumidity.

When the fifth crossleak occurrence condition is satisfied, thedifference between the anode gas partial pressure on the anode electrodeside and that on the cathode electrode side is large; therefore,crossleak is likely to occur. The threshold value can be empiricallydetermined in advance. For example, a value in the range of 20 kPa to 30kPa may be used as the threshold value.

The above conditions may be combined as appropriate, to be used as acrossleak occurrence condition. In this embodiment, the first crossleakoccurrence condition is used.

In step S200 of FIG. 3, when the crossleak occurrence condition is notsatisfied, the controller 20 proceeds to step S210, to determine areference target voltage Vref as the target voltage Vm. The referencetarget voltage Vref can be empirically determined in advance, as avoltage that permits the fuel cell system 100 to return from the zerorequired output operation to the normal operation, without a responsedelay, when power is required to be generated. On the other hand, whenthe crossleak occurrence condition is satisfied, the controller 20proceeds to step S215, to determine a value Vup that is higher than thereference target voltage Vref, as the target voltage Vm.

The timing chart of FIG. 4 shows one example of the relationship amongthe anode gas pressure, average cell voltage Vfc, and the indicated flowrate of cathode gas. A graph in the upper section indicates change ofthe anode gas pressure, and a graph in the middle section indicateschanges of the average cell voltage Vfc and the target voltage Vm. Agraph in the lower section indicates change of the indicated flow rate(which will be referred to as “cathode gas indicated flow rate”) of theflow rate of cathode gas supplied. As shown in FIG. 4, the controller 20switches the fuel cell system 100 from normal operation to zero requiredoutput operation, at time t0. In the example of FIG. 4, power generationis stopped in the fuel cell stack 10, and the cathode gas indicated flowrate is set to zero. When the cathode gas indicated flow rate is equalto zero, operation of the air compressor 33 is stopped.

As shown in the graph in the upper section of FIG. 4, the anode gaspressure gradually decreases, due to crossleak of anode gas, from timet0 at which the fuel cell system 100 switched to zero required outputoperation. Then, at time t2, the anode gas pressure is reduced to athreshold pressure Pt based on which it is determined whether thecrossleak occurrence condition is satisfied.

As shown in the graph in the middle section of FIG. 4, when the fuelcell system 100 switches from normal operation to zero required outputoperation, in a situation where crossleak is likely to occur, theaverage cell voltage Vfc is rapidly reduced. If the average cell voltageVfc is excessively reduced, the performance of the unit cell 11 maydeteriorate. Therefore, in the example of FIG. 4, at time t1 at whichthe average cell voltage Vfc becomes lower than the target voltage Vm,the controller 20 resumes supply of cathode gas to the fuel cell stack10 by use of the air compressor 33. Thus, the average cell voltage Vfccan be made less likely or unlikely to be excessively reduced, bycontrolling supply of cathode gas, more specifically, by setting thetarget voltage Vm to the value Vup higher than the reference targetvoltage Vref, so that supply of cathode gas is started at an earlierpoint in time t1.

As shown in the graph in the lower section of FIG. 4, supply of cathodegas is stopped in a period from time t0 to time t1; thus the indicatedair flow rate is set to zero. The controller 20 executes step S130 ofFIG. 2 (resumes cathode gas supply in the example of FIG. 4) at time t1,and executes control of steps S100 to step S140 in each control cycle.As a result, the flow rate of cathode gas supplied is intermittentlyincreased. After time t2, the crossleak occurrence condition is notsatisfied; therefore, in step S110 of FIG. 2, the target voltage Vm isset to the reference target voltage Vref, and control for intermittentlyincreasing the flow rate of cathode gas supplied is performed. In thisexample of FIG. 4, the indicated flow rate of cathode gas after time t2at which the crossleak occurrence condition ceases to be satisfied isset to a value smaller than the cathode gas indicated flow rate Q1 inthe period from time t1 to time t2, in which the crossleak occurrencecondition is satisfied. However, the disclosure is not limited to this,but the cathode gas indicated flow rate may be determined as desired.

According to the fuel cell system 100 of this embodiment, when thecrossleak occurrence condition is satisfied, the controller 20 sets thetarget voltage Vm to a value higher than that in the case where thecrossleak occurrence condition is not satisfied, and performs cathodegas supply control for increasing the average cell voltage Vfc.Therefore, the point in time at which the flow rate of cathode gassupplied is increased is advanced (i.e., the flow rate of cathode gas isincreased at the earlier time), and the cell voltage is less likely orunlikely to be reduced excessively.

Also, the crossleak occurrence condition used in this embodiment is thatthe anode gas pressure is higher than the predetermined thresholdpressure; therefore, occurrence of crossleak can be easily detected.

B. Second Embodiment

The flowchart of FIG. 5 illustrates one example of the procedure ofcathode gas supply control according to a second embodiment. Theconfiguration of a fuel cell system of the second embodiment isidentical with that of the fuel cell system of the first embodiment, andwill not be described herein. The cathode gas supply control of thesecond embodiment is different from that of the first embodiment in thata liquid water purge process is performed when liquid water isaccumulated, but is identical with that of the first embodiment in theother steps. The liquid water purge process is a scavenging process forreducing water that remains in the fuel cell stack 10, and waterdeposited on pipes, valves, etc. of the fuel cell system 100, bycontrolling respective constituent parts of the fuel cell system 100.

In the above first embodiment, after determining that the average cellvoltage Vfc is lower than the target voltage Vm in step S140, thecontroller 20 returns to step S140, and continues to supply cathode gas.In the second embodiment, after determining that the average cellvoltage Vfc is lower than the target voltage Vm in step S140, thecontroller 20 performs the liquid water purge process as needed,according to steps S150, S160.

In step S140, the controller 20 obtains the average cell voltage Vfcagain. When the obtained average cell voltage Vfc is equal to or lowerthan the target voltage Vm, the controller 20 proceeds to step S150, anddetermines whether the average cell voltage Vfc obtained in step S140 islower than the lower-limit voltage Vlow. The lower-limit voltage Vlowcan be empirically determined in advance, as a voltage value based onwhich it is determined that liquid water is accumulated in the fuel cellstack 10 (for example, a voltage value at which the catalyst of the unitcell 11 switches between oxidation and reduction). The lower-limitvoltage Vlow is a value equal to or higher than 0.4V and equal to orlower than 0.7V, and is set to a value lower than the target voltage Vmused in step S140. When the average cell voltage Vfc is lower than thelower-limit voltage Vlow, the controller 20 proceeds to step S160, andperforms the liquid water purge process. On the other hand, when theaverage cell voltage Vfc is equal to or higher than the lower-limitvoltage Vlow, the controller 20 returns to step S140, and continues tosupply cathode gas.

After performing the liquid water purge process in step S160, thecontroller 20 returns to step S100. In the liquid water purge process ofthis embodiment, the controller 20 controls the cathode gas supply unit30 to excessively inject cathode gas into the fuel cell stack 10. Forexample, it is preferable to inject cathode gas for several seconds at aflow rate that is 10 times as large as the flow rate necessary tomaintain the normal voltage, more specifically, the cathode gasindicated flow rate after time t2 when the crossleak occurrencecondition is not satisfied, as shown in FIG. 4.

According to the fuel cell system 100 of this embodiment as describedabove, the controller 20 performs the purge process when accumulation ofliquid water occurs. Thus, the cell voltage can be made less likely orunlikely to be excessively reduced.

C. Other Embodiments

The configuration of a fuel cell system of a third embodiment asdescribed below is identical with that of the fuel cell system of thefirst embodiment, and thus will not be described herein. The cathode gassupply control of the third embodiment is different from that of thefirst embodiment in the relationship between the anode gas pressure andthe target voltage Vm in the target voltage determining process, but isidentical with that of the first embodiment in the other steps.

The graph of FIG. 6 shows the relationship between the anode gaspressure and the target voltage Vm used in the target voltagedetermining process in the third embodiment. In this example, when theanode gas pressure is equal to or higher than the threshold pressure Ptat or above which the crossleak occurrence condition is satisfied, theslope of reduction of the average cell voltage Vfc immediately afterswitching to the zero required output operation is steeper as the anodegas pressure is higher; therefore, the target voltage Vm is set to alarger value as the anode gas pressure is higher. While the targetvoltage Vm is set to one of three values selected according to the anodegas pressure in the third embodiment, the target voltage Vm may be setin a different manner. With the fuel cell system 100 thus configured,since the target voltage Vm is set to a larger value as the anode gaspressure increases, the cell voltage can be effectively made less likelyor unlikely to be excessively reduced.

The graph of FIG. 7 shows the relationship between the anode gaspressure and the target voltage Vm used in the target voltagedetermining process according to a fourth embodiment. This example issimilar to that of FIG. 6 in that the target voltage Vm is set to alarger value as the anode gas pressure is higher, but the relationshipbetween the anode gas pressure and the target voltage Vm is representedby a continuous curve. With this configuration, too, since the targetvoltage Vm is set so as to increase as the anode gas pressure increases,the cell voltage can be made less likely or unlikely to be excessivelyreduced.

The graph of FIG. 8 shows the relationship between the anode gaspressure and the target voltage Vm used in the target voltagedetermining process according to a fifth embodiment. This example issimilar to that of FIG. 6 in that the target voltage Vm is set to alarger value as the anode gas pressure is higher, but the relationshipbetween the anode gas pressure and the target voltage Vm is determinedaccording to the temperature of the fuel cell stack 10. In FIG. 8, graphG1 a indicates the relationship between the anode gas pressure and thetarget voltage Vm when the temperature of the fuel cell stack 10 ishigh, and graph G1 b indicates the relationship between the anode gaspressure and the target voltage Vm when the temperature of the fuel cellstack 10 is low. A criterial temperature based on which it is determinedwhether the temperature of the fuel cell stack 10 is high or low isempirically determined in advance, and may be determined as desired.Comparison between the graph G1 a and the graph G1 b finds that thetarget voltage Vm of the graph G1 b is lower than that of the graph G1a. Thus, the target voltage Vm is set to a higher value when thetemperature of the fuel cell stack 10 is high, than that in the casewhere the temperature is low. The graph G1 a and the graph G1 b may beidentical with each other when the anode gas pressure is equal to orlower than the threshold pressure Pt. With this configuration, too, thetarget voltage Vm is set so as to increase as the anode gas pressureincreases, in the range in which the crossleak occurrence condition issatisfied, so that the cell voltage can be effectively made less likelyor unlikely to be excessively reduced.

The graph of FIG. 9 shows the relationship between the anode gaspressure and the target voltage Vm used in the target voltagedetermining process according to a sixth embodiment. This example issimilar to that of FIG. 8 in that the relationship between the anode gaspressure and the target voltage Vm is determined according to thetemperature of the fuel cell stack 10, and the target voltage Vm is setto a larger value as the anode gas pressure is higher. However, in theexample of FIG. 9, threshold values of the anode gas pressure at whichthe value of the target voltage Vm changes are set depending on thetemperature of the fuel cell stack 10. In FIG. 9, graph G2 a indicatesthe relationship between the anode gas pressure and the target voltageVm when the temperature of the fuel cell stack 10 is high, and graph G2b indicates the relationship between the anode gas pressure and thetarget voltage Vm when the temperature of the fuel cell stack 10 is low.Comparison between the graph G2 a and the graph G2 b finds that thethreshold values of the anode gas pressure at which the value of thetarget voltage Vm changes are larger in the graph G2 b, than those inthe graph G2 a. For example, the anode gas pressure as a threshold valueof the crossleak occurrence condition is Pt1 in the graph G2 a, and isPt2 that is higher than Pt1, in the graph G2 b. Also, the target voltageVm is lower in the graph G2 b. With this configuration, too, the targetvoltage Vm is set so as to increase as the anode gas pressure increases,in the range in which the crossleak occurrence condition is satisfied,so that the cell voltage can be effectively made less likely or unlikelyto be excessively reduced.

In the embodiments shown in FIG. 8 and FIG. 9, the target voltagedetermining process is performed using the graphs in which therelationship between the anode gas pressure and the target voltage Vmdiffers according to the temperature of the fuel cell stack. Instead,the target voltage determining process may be performed using a graph inwhich the relationship between the anode gas pressure and the targetvoltage Vm differs, according to the relative humidity of theelectrolyte membrane of each unit cell 11 of the fuel cell stack 10.More specifically, the target voltage Vm is set to a higher value whenthe relative humidity of the electrolyte membrane of each unit cell 11of the fuel cell stack 10 is high, than that in the case where therelative humidity is low.

The present disclosure is not limited to the above embodiments, but maybe implemented with various configurations, without departing from thescope thereof. For example, technical features in the embodiments, whichcorrespond to technical features described in “SUMMARY”, may be replacedwith other features or combined as appropriate, so as to solve a part orthe whole of the problems mentioned above, or achieve a part or thewhole of the effects mentioned above. If there is any technical featurethat is not described as being essential in this specification, thetechnical feature may be deleted as appropriate.

What is claimed is:
 1. A fuel cell system comprising: a fuel cell stackhaving a plurality of unit cells; an anode gas supply unit that suppliesanode gas to the fuel cell stack; a cathode gas supply unit thatsupplies cathode gas to the fuel cell stack; a voltage detector thatdetects a voltage of the fuel cell stack; and a controller that controlsthe anode gas supply unit and the cathode gas supply unit, wherein thecontroller performs cathode gas supply control to raise an average cellvoltage of the fuel cell stack by increasing supply of the cathode gasto the fuel cell stack by the cathode gas supply unit, when electricpower required to be generated by the fuel cell stack is equal to zero,and the average cell voltage is lower than a predetermined targetvoltage, and under the cathode gas supply control, the controllerdetermines whether a predetermined condition indicating that crossleakis likely to occur is satisfied, and sets the target voltage when thepredetermined condition is satisfied, to a value that is higher than areference target voltage as the target voltage in a case where thepredetermined condition is not satisfied, the crossleak representingpermeation of the anode gas from an anode electrode to a cathodeelectrode in each of the unit cells.
 2. The fuel cell system accordingto claim 1, further comprising a pressure measuring unit that measuresan anode gas pressure of the fuel cell stack, wherein the predeterminedcondition comprises a condition that the anode gas pressure is higherthan a predetermined threshold pressure.
 3. The fuel cell systemaccording to claim 2, wherein, under the cathode gas supply control, thecontroller sets the target voltage to a higher value when the anode gaspressure is higher than the threshold pressure, than the target voltagein a case where the anode gas pressure is lower than the thresholdpressure.
 4. The fuel cell system according to claim 1, wherein thepredetermined condition comprises a condition that the electric powerrequired to be generated by the fuel cell stack immediately before therequired electric power is reduced to zero is equal to or large than apredetermined threshold power.
 5. A method of controlling a fuel cellsystem having a fuel cell stack having a plurality of unit cells, themethod comprising: performing cathode gas supply control to raise anaverage cell voltage of the fuel cell stack by increasing supply ofcathode gas to the fuel cell stack, when electric power required to begenerated by the fuel cell stack is equal to zero, and the average cellvoltage is lower than a predetermined target voltage, wherein, under thecathode gas supply control, it is determined whether a predeterminedcondition indicating that crossleak is likely to occur is satisfied, andthe target voltage is set when the predetermined condition is satisfied,to a value that is higher than a reference target voltage as the targetvoltage in a case where the predetermined condition is not satisfied,the crossleak representing permeation of anode gas from an anodeelectrode to a cathode electrode in each of the unit cells of the fuelcell stack.